High magnification achromatic microscope objective having a wide, flat field

ABSTRACT

A high magnification microscope objective which has a wide field of about the field number of 30; in which the curvature of the field is sufficiently eliminated and the in-axis chromatic aberration is completely eliminated to the extent which is achieved by an apochromatic objective.

Inventor Appl. No. Filed Patented Assignee Priority HIGH MAGNIFICATIONACIIROMATIC MICROSCOPE OBJECTIVE HAVING A WIDE,

References Cited UNITED STATES PATENTS 1 H1944 Bennett 3/ 1965 Ruben3,380,793 4/1968 Klein. 3,410,633 11/1968 Young Primary Examiner-John K.Corbin Attorney-Otto John Munz ABSTRACT: A high magnification microscopeobjective which has a wide field of about the field number of 30; inwhich the curvature of the field is sufficiently eliminated and theinaxis chromatic aberration is completely eliminated to the extent whichis achieved by an apochromatic objective.

FLAT FIELD 1 Claim, 2 Drawing Figs. 0.8. CI 350/177, 350/214, 350/215,350/234 Int. Cl G021) 9/64, G02b 21/02 1 r r5 r r 1'2 1'4 1'7 1' IF T 2P dl H n2 c0 7 n a "7 1 "I1 012 4 2 d d5 "9 d9 d PATENTEUHARBOIQYI3572.902

' SHEET 1 [IF 2 INVENTOR TOSHIFUMI UE'I'AKE HIGH MAGNIFICATIONACIIROMATIC MICROSCOPE OBJECTIVE HAVING A WIDE, FLAT FIELD BACKGROUND OFTHE INVENTION The present invention relates to an objective for amicroscope, and more particularly, to an object for a microscope whichhas a high magnifying power such as ranging between about 20 X and aboutl X and a very long durability without deteriorating its performance andin which the curvature of the field is sufficiently eliminated over theultrawide field such as the field number of 30 and, at the same time,the in-axis chromatic aberration is eliminated almost completely to theextent which is achieved by an apochromatic objective over the very wideaperture.

An objective of the type described above is very difficult in design aswell as in manufacture, and very few objectives having superior qualityhave appeared in the market. Presently, plan-achromatic objectives havebeen in the market as the objectives of a microscope which can produceflat image surface over the wide field. The flatness of the imagesurface obtained by such plan-achromatic objectives is at the highest ina degree in which an eyepiece usually used in combination and having thefield number of about 20 can be used together with the plan-achromaticobjectives, and, therefore, it cannot be said that they aresatisfactory. Further, they are constructed of conventional opticalglasses, and the secondary spectrum of the in-axis chromatic aberrationcannot be eliminated although the first spectrum thereof is eliminated,and, therefore, the chromatic aberration still remains partially. Quiterecently, a plan-apochromatic objective is appearing in the market whichcan completely eliminate the secondary spectrum of the in-axis chromaticaberration. In general, it has been the common practice for removing thesecondary spectrum of the inaxis chromatic aberration caused in anobjective of the class of the apochromatic objective to use a crystal ofalum as the material for making the concave lens element in theobjective and 'a crystal of fluorite as the material for making theconvex lens element, both the alum and the fluorite being of theisometric system. The reason for the above is due to the facts that boththe above crystals very closely resemble each other in the nature of thepartial dispersion ration thereof and they both have the characteristicsby which the difference in the spherical aberration caused by thedifference in color is difficult to take place. However, the alum isvery expensive and, at the same time, it is very weak in mechanicalstrength thereby making the working of the alum very difficult. Further,the alum is not stable chemically, and, therefore, it will soon besubjected to deliquescence when it is left in the atmosphere, or ittends to lose the water of crystallization therefrom so that thedevitrification will be effected. From the past, an apochromaticobjective was said to be subjected to the devitrification within severalyears even though it was carefully maintained, thereby making theobjective useless. This is the fatal defect of the prior artapochromatic objectives. This is chiefly due to the chemical instabilityof the alum crystal. In contrast to the above, a crystal of fluorite isrelatively stable chemically, and semiapochromatic objectivesincorporating crystal of fluorite in the lens system thereof have beenin the market. They are inexpensive and, at the same, they have longdurability. Therefore, they have been appreciated by the consumers incomparison with the apochromatic objectives, despite the fact that thecorrection of the chromatic aberration obtained by the semiapochromaticobjectives is not sufficient in comparison with that obtained by theapochromatic objectives.

However, it is very difficult to design high-quality apochromaticobjectives by using the fluorite only. Further, in a planapochromaticobjective of the type in which a flat image surface is to be obtained,it is necessary to sufficiently correct the curvature of the field aswell as the coma over the ultrawide field while the secondary spectrumof the in-axis chromatic aberration is eliminated, thus making thedesigning of the plan-apochromatic objective extremely difficult. Thedesigning of a plan-apochromatic objective having a high magnifyingpower such as 40X, IOOX and the like is particularly dif ficult becauseof the greater numerical aperture thereof.

SUMMARY OF THE INVENTION Therefore, the object of the present inventionis to provide a planapochromatic objective having a high magnifyingpower by using fluorite.

A further object of the present invention is to provide aplan-apochromatic objective of the type described above which has amagnifying power such as ranging between about 20X and about [00X and avery long durability without deteriorating its quality and in which thecurvature of the field is sufficiently eliminated over the ultrawidefield such as the field number of 30 and, at the same time, the in-axischromatic aberration is eliminated almost completely over the very wideaperture.

These objects are achieved by an objective constructed in accordancewith the present invention, which is characterized by:

A. a leading lens element at the object side in the form of a thickconcave meniscus, the refractive power of the air contacting surfacethereof which faces to the object satisfying the following condition:

where:

r= the radius of curvature of the air contacting surface of the leadinglens element,

F the focal length of the entire lens system,

n the refractive index of the leading lens element with respect to dline;

B. a large air gap provided in the lens system, the amount of the airgap satisfying the following condition:

g. 8.0 F 2.0 where:

d= the amount of the air gap.

C. the rearmost lens element in the forward lens group in the lenssystem which is separated by the air gap from the rearward lens group ofthe lens system being in the form of a convex meniscus, the convexsurface of which is directed to face to the object, the focal length ofthe rearmost lens element satisfying the following condition:

where:

f the focal length of the rearmost lens element;

D. cemented achromatic surfaces, if provided in the lens system, theconvex side of the cemented surfaces being arranged to face against theobject;

E. the rearward lens group of the lens system being in the fon'n of aconvex meniscus as a whole, the first air contacting surface at theobject side and the rearmost air contacting surface at the image side inthe rearward lens group being arranged such that the convex sidesthereof are directed toward the image side respectively;

F. the resultant focal length of the rearward lens group as a wholesatisfying the following condition:

where:

f the resultant focal length of the rearward lens group; G. at least twoachromatic surfaces being provided in each of the forward and rearwardlens groups of the lens system. The physical function and effectivenessof each of the features of the above-described objective of the presentinvention will be described in detail hereinbelow.

CONDITION A Arrangement of the leading lens element in the form of thickconcave meniscus coaxial with respect to the object is an indispensablecondition for eliminating the spherical aberration as well as the comain an objective having a large aperture. To select the radius ofcurvature r of the first surface of the leading lens element within theabove-described range of the numerical value is necessary to completelyeliminate the curvature of the field of the entire lens system over theultrawide field such as reaching the field number of 30. In case therefractive power (n-l )/r of the first surface is less than theabove-described lower limit 1.0, serious undercorrection of theastigmatism will take place, thus making it impossible to correct theastigmatism by the corrective power obtainable from the lens elementslocated rearwardly of the leading lens element. Further, the annularband aberration clue to the spherical aberration will increase. Also,the working distance, i.e. the distance between the cover glass and theair contacting surface of the objective at the object side thereof willbe made shorter thereby making the operation of the microscopeinconvenient. On the other hand, if the refractive power exceeds theabove-described upper limit 2.5, the corrective power for the curvatureof the field will be made insufficient, so that a sharp image cannot beformed over the ultrawide field such as reaching the field number of 30.

CONDITION B This condition B is provided in order to maintain thebalance between the conditions for the correction of the inaxischromatic aberration and for the correction of the chromatic aberrationdue to the magnification in the entire lens system of the objective. Andthis condition is indispensable for the correction of the coma in theentire lens system, particularly for the correction of the coma in theoff-axis light bundle at the ultrawide visual angle taking place in therange of field number of 30. If the value d of the air gap is made lessthan the set lower limit, the in-axis chromatic aberration will bedeteriorated thereby making the condition for correction of the inaxischromatic aberration insufiicient for an apochromatic objective. And theunsymmetry in the coma at the ultrawide visual angle in the range of thefield number of 30 cannot be eliminated. n the other hand, if the valueof the air gap exceeds the above-described upper limit, the degree ofundercorrection of the chromatic aberration due to the magnificationwill be made serious thereby making it impossible to correct the sameeven though a corrective eyepiece is used in combination. Further, theinterception of the off-axis light bundle at the wide visual angle willbe made greater so that the light quantity at the marginal portion ofthe image will be made insufficient.

CONDITION C This condition C is provided in order to maintain theworking distance of the entire lens system in an appropriate value. Ifthe value fl/F is made less than the above-described lower limit 10, thecorrective power for the annular band aberration due to the sphericalaberration in the forward lens group will be deteriorated.

CONDITION D This condition D is provided in order to maintain thebalance in the coma and the spherical aberration in the forward lensgroup as well as the in-axis chromatic aberration. If the convex side ofthe cemented achromatic surfaces in the lens system is arranged to faceto the image side, serious deterioration in the coma and the sphericalaberration will take place.

CONDITION E This condition E is provided in order to eliminatecompletely the residual aberration of the spherical aberration and thecoma resulting from the leading lens element, i.e. the thick concavemeniscus in the forward lens group and the convex meniscus located atthe end of the forward lens group from the lens system of the objectivein its entirety. If the abovereferred convex surfaces in the rearwardlens group are directed to face to the object side as a whole, then thecorrective power for the respective above-recited aberrations will belost.

CONDITION F This condition F is provided in order to completelyeliminate the in-axis chromatic aberration even the secondary spectrumthereof in cooperation with the following condition G. If the value f,/Fis made less than the set lower limit 5.0, then the chromatic aberrationdue to the spherical aberration in the shorter wavelength caused by theovercorrection cannot be completely eliminated.

CONDITION G Since an objective such as that of the present inventionincorporates in the lens system meniscus lens elements having greatercurvature in order to completely eliminate the curvature of the fieldover the ultrawide range of the field, serious undercorrection of thechromatic aberration due to magnification will necessarily take place bythe provision of such meniscus lens elements. And, when the chromaticaberration due to the magnification is to be corrected, the in-axischromatic aberration which is incompatible to the chromatic aberrationdue to the magnification will necessarily be deteriorated.

The condition G solves the above difficulty by providing at least twocemented achromatic surfaces in each of the forward and rearward lensgroups so that the in-axis chromatic aberration is corrected to thedegree attainable by an apochromatic objective.

Now an embodiment of the present invention will be described below inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a longitudinal sectionalview showing the construction of the embodiment of the objective inaccordance with the present invention; and

FIG. 2 shows various graphs of the various aberration curves obtained bythe objective of FIG. I.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. I, theembodiment of the objective of the present invention comprises a forwardlens group consisting of six lens elements of which the second and thirdas well as the fifth and sixth lens elements from the object side arecemented with each other and a rearward lens group spaced a greaterdistance from the forward lens group by an air gap d which rearward lensgroup consists of four lens elements of which the second, third andfourth lens elements from the object side are cemented with each other.A slide glass 86 and a cover glass CG sandwiching therebetween theobject P are located in front of the leading lens element of the forwardlens group of the objective.

The radius of curvature of each of the lens elements and the slide glassas well as the cover glass and the air gaps between the respectiveadjacent two elements are designated as shown.

The radius of curvature r, of the air contacting surface of the leadinglens element in the form of a thick concave meniscus is set so as tosatisfy the following condition.

where:

F the focal length of the entire lens system of the objective,

n =the refractive index of the leading lens element with respect to dline. The above-described large air gap 11,, is set so as to satisfyingthe following condition:

Following the requirements of the present invention described above, thelast lens element, i.e. the sixth lens element in the forward lens groupis made a convex meniscus, the convex surfaces of which is arranged toface to the object.

The last lens element is constructed so that the focal length f thereofsatisfies the following condition:

In the embodiment shown, a cemented achromatic surface is provided inthe above-described last lens element, the convex side of which isarranged to face to the object.

Also, the first air contacting surface at the side of the object and therearmost air contacting surface of the rearward lens group are soarranged in accordance with the above-described requirements of thepresent invention that the convex sides of the above-described aircontacting surfaces are directed toward the image side so that therearward lens group is made in the form of a convex meniscus as a whole.

The resultant focal length f, of the rearward lens group as a whole isset so as to satisfy the following condition:

TABLE r (1 nd Vd 0. 17 1 m 0. 209 1. 5229 59. 9 -1.26 2. 80 1 -2. 210.05 1.433 81. 5 ---14, 4 0.40 1 47. 6 2.10 1. 613 43. 9 -3. 88 0.191.434 95.2 29. 5 1. 60 1 7. 71 0.10 1.434 95. 2 12. 6 0.80 I 4. 99 2. 201.613 43. I 43. 6 13.0 1.434 95. 2 111. 1. 50 1 Remarks to the abovetable:

Magnification 40.0X Numerical Aperture N.A. 0.85 F 4:20 Petzval sum=0.03(This value is in the case of F=l .0)

As seen from the above, the' Petzval sum of the objective of the presentinvention is very small in contrast to the value of about l.3 which isobtained by the prior art apochromatic objective. And the numericalaperture N.A. of the objective of the present invention is sufficientlylarge.

The prior art objective of the magnification of 40X can provide thenumerical value of only about 0.65. A prior art semiapochromaticobjective can provide the numerical value of only about 0.75. This showsthat a very light image can be obtained by the objective of the presentinvention.

FIG. 2 shows the various aberration curves of the embodiment of theobjective of the present invention, and FIG. Z-a shows the sphericalaberration curves, FIG. 2-!) shows the sine-condition curves (OSC'), thesolid lines being the aberration with respect to d line, dotted linesbeing the aberration with respect to c line, while the broken lines arethe aberration with respect to F line and one-dot chain lines are theaberration with respect to 8 line.

As seen from FIG. 2-0, the spherical aberration of the objective of thepresent invention is very small, ranging within only :2 mm. The OSC'curve for 3 line shown in FIG. Z-b is relatively large. However, sincethe 8 line is a purple color having the wavelength near the ultravioletregion, the visibility factor of the g line is relatively low.Therefore, the relatively large deviation of the sine-condition withrespect to the 3 line has practically no effect FIG. 2-c shows theastigmatism of the embodiment of the objective of the present invention.As is clear from FIG. 2-c the astigmatism is within the range of 2mm.over the wide field such as reaching the field number of 30. FIG. 2-dshows the distortion of the embodiment of the objective of the presentinvention. The distortion in FIG. 2-11 is relatively large; however, thedistortion is not serious in an objective for a microscope, since thedistortion can be corrected by means of the eyepiece used in combinationtherewith. FIG. 2-e shows the coma appearing in the objectives havingthe field numbers 13, 18.2, 26 and 30, respectively. As is clear fromthese coma curves, the coma can be held to very small values in anobjective constructed in accordance with the present invention. Althoughthe coma is relatively large in the range near the field number of 30,the amount of such coma is widely improved in comparison with that ofthe prior art objectives.

As described above, in accordance with the present invention, it isclear that, in an objective of the present invention, the curvature ofthe field is sufficiently improved over the ultrawide field such asreaching the field number of 30, and a flat image surface is obtained,and, further, the second spectrum of the chromatic aberration can becompletely eliminated over the sufficiently large aperture as isobtainable by an apochromatic objective.

Since the objective of the present invention utilizes only the fluoriteinstead of the alum, the designing and the manufacture of the objectiveof the present invention are made very easy thereby pennitting the costfor production to be made very low while the durability of the same isvery superior without deteriorating the performance thereof.

It should be understood that the present invention is not limited to theparticular embodiment as described above but includes broadly all themodifications and variations which fall within the scope and spirit ofthe present invention as defined in the appended claims.

Iclaim:

l. A microscope objective comprising:

a forward lens group consisting of six lens elements of which the secondand third lens elements from the object side are cemented, and the fifthand sixth lens elements from the object side are cemented; and

a rearward lens group consisting of:

four lens elements of which the 'second, third, and fourth lens elementsfrom the object side are cemented, said lens elements havingsubstantially the numerical data set forth in the following table:

r (1 nd W1 m 0. 209 1. 5229 59. 9 1.26 2.80 1 2. 21 0. 05 l. 486 81. 5-14. 4 0. 40 47. 6 2.10 1. 613 43. 9 --3. 88 0. 19 1. 434 95. 2 29. 5 1.60 l 7 8 where: jective with respect to d line, and

r= the respective radius of curvature of the elements of the Vd Abbe'snumber of each of the elements of the objecobjective, tive with respectto d line d the amount of the air gap between the respective adthenumerals designating the order of the arrangement from jacent twoelements of the objective, the object side.

nd the refractive index of each of the elements of the ob- UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,572,902 Dated Mar.30, 1971 Inventor) Toshif umi Uetake It is certified that error appearsin the above-identified patent and that said Letters Patent are herebycorrected as shown below:

On the cover sheet [54] "ACHROMATIC" should read APOCHROMATIC Signed andsealed this 18th day of April 1972.

(SEAL) Attest:

ROBERT GOTTSCHALK EDWARD M.FLETCHER,JR.

Commissioner of Paten Attesting Officer

1. A microscope objective comprising: a forward lens group consisting ofsix lens elements of which the second and third lens elements from theobject side are cemented, and the fifth and sixth lens elements from theobject side are cemented; and a rearward lens group consisting of: fourlens elements of which the second, third, and fourth lens elements fromthe object side are cemented, said lens elements having substantiallythe numerical data set forth in the following table: